In transducer circuitry for electrical power metering, it is often desired to provide an output signal current which is proportional to an input parameter such as input voltage or current or real or reactive power in order to use that output current for metering or subsequent operations.
Typically this is accomplished by connecting the primary of a step-down transformer in the electrical power circuitry to sense instantaneous voltage or current and provide a relatively low power voltage or current signal at its secondary. Such a voltage or current transformer scales the high voltages and currents of the electrical power circuitry, down to lower levels for use by small signal circuitry. The scaled signal is then conditioned by intermediate circuitry, the function of which usually includes conversion to a DC signal which is proportional to the input parameter which it is desired to meter. This DC signal is then applied to an output stage.
For example, the conditioning and conversion to a DC signal might be accomplished by rectifying and filtering the signal from the secondary of a voltage or current transformer to derive a DC signal which is proportional to the input voltage or current. Alternatively, the conditioning can involve a conventional multiplier circuit having both an input voltage signal and an input current signal for deriving the real power in watts or the reactive power in vats.
It is desirable that the output stage amplifier have a high output impedance so that it operates as a current source in which the output signal is dependent upon the input parameter represented by the DC signal, but not dependent upon the impedance of the load which it drives.
Conventional op-amp amplifiers typically used as the output stage are limited, however, in their operation by their compliance voltage. The compliance voltage of such an amplifier is the absolute value of the maximum output voltage to which the output of the op amp can go and still provide an accurate output current which is a linear function of the input signal. Thus, the compliance voltage is the maximum voltage at which the output still appears as a current source, accurately obeying the desired transfer function for the amplifier.
Therefore, the product of the maximum or full scale output current for the design range of operation, multiplied by the maximum load impedance which may be used can not exceed the compliance voltage. Consequently, the compliance voltage imposes a limitation upon the maximum load impedance and the maximum load current which may be used.
In the operational amplifier circuits which have been used by the prior art, the compliance voltage is limited to a voltage which is less than the power supply voltage. Typically, for example, an amplifier having a power supply voltage of .+-.15 volts will have a compliance voltage on the order of .+-.10 or .+-.11 volts.
It is an object and feature of the present invention to provide an operational amplifier circuit which substantially increases, nearly doubling, the compliance voltage. This increase in the compliance voltage permits an increase in the current range or load impedance or both with which this circuit may be operated.